1. Field of the Invention
The present invention relates to a revolving-flux type motor provided with a stator which includes a coil wound around an iron core and a rotor which includes permanent magnets fixed to an iron core.
2. Description of the Prior Art
FIGS. 5(a) and 5(b) illustrates a rotor for a revolving-flux type motor of the prior art, and FIGS. 6(a), 6(b) and 6(c) illustrate structures for fixing permanent magnets using a holder band.
In the rotor for a revolving-flux type motor provided with a stator including a coil wound around an iron core and a rotor including permanent magnets fixed to an iron core, each permanent magnet attached to the rotor is generally of an arcuate shape. If the magnet has a uniform thickness, i.e. uniform thickness radially of the rotor, the distribution of magnetic flux density at the gap between the stator and the rotor forms a square waveform so that the efficiency of the motor is reduced due to the influence of harmonic components exceeding the fundamental wave of a sinusoidal waveform.
In order to obtain the flux density distribution of a sinusoidal waveform, corner portions of the arc-shaped permanent magnets are cut or recessed in the prior art, or, as shown in FIG. 5(a), permanent magnets 82 having eccentric outer peripheries are fixed to a cylindrical rotor iron core 81 so as to form a magnetic circuit as shown in FIG. 5(b). In this manner, the flux density distribution at the gap 85 between the permanent magnets 82 and the iron core 83 of the stator has a sinusoidal waveform in the lateral direction of the permanent magnets.
High speed motors generally employ band clamps for fixing permanent magnets to the iron cores of the rotors, an example of which is illustrated in FIGS. 6(a), 6(b) and 6(c) which are top, elevation and cross-section views, respectively.
As shown in FIGS. 6(a), 6(b) and 6(c), a band clamp device includes an endless band 86 formed of a cylindrical sheet, retainers 87 for tightening the band 86 at spaces between the permanent magnets 82 and screws for fixing the retainers 87 to the iron core 81 of the rotor. For fixing the permanent magnets 82 to the iron core 81, the permanent magnets 82 are initially positioned and aligned on the outer periphery of the iron core 81, and are covered with the band 86. Then, the retainers 87 are located on portions of the band 86 over the joints between the permanent magnets 82 and are fastened to the iron core 81 by the screws 88, so that the band 86 and the retainers 87 are moved into the joints between the permanent magnets 82 to fix the permanent magnets 82.
However, the above described manner of using a band clamp to fix the permanent magnets 82 to the iron core 81 requires time-consuming operations to locate the permanent magnets and fix them with the band clamp, so that productivity is low.
In order to eliminate these time-consuming operations and to increase productivity, automatic assembling of the permanent magnets on the iron core by an endless band clamp has been proposed. However, if the permanent magnets have cut or recessed corners or eccentric outer peripheries, the band clamp cannot have a simple cylindrical form and, therefore, fitting the band on the rotor becomes difficult. Further, this approach also reduces contact area between the permanent magnets and the band, so that particularly large loads must be applied to portions of the band contacting the magnets and thus the tightening effect is reduced.
It is an object of the invention to provide a design overcoming the above-noted problems, which design enables automatic assembling of the permanent magnets by use of a band clamp. It is another object of the invention to tightly fix the permanent magnets with a sufficiently large force by band clamping.